Electrolysis is the chemical process which takes place at the electrodes when direct current is passed through an electrolyte in contact with the electrodes. More specific, compounds which are dissociated into ions in the electrolyte is reduced at the cathode and oxidized at the anode, by means of an applied current. One of the most important electrolysis processes is electrolysis of alumina solved in a melted halogenide electrolysis bath, for example an electrolysis bath of cryolite. The process which is utilized when producing aluminium the Hall-Heroult-process which was invented simultaneously and independently by the American Hall and the Frenchman Heroult, is about one hundred years old and has not been developed flier as far as other processes of electrolysis. This is probably due to the harsh conditions which are required to perform electrolysis and to keep the electrolysis bath in an operative condition, for example a temperature in the electrolyte up to 980° C.
By electrolysis, and in particular by electrolysis of alumina for production of aluminium, a significant loss is present in the form of a reduced current efficiency and loss of heat, and for production of aluminium the energy cost is a very significant part of the total cost. Technology which could provide better current efficiency would lead to significant savings. This problem is general when it comes to electrolysis, and the invention is in general applicable for the electrolysis industry, in addition to that it could be applicable within other industrial fields where corresponding problems are found, for example in other energy consuming industry and within the energy network However, the present invention is in particular focused on aluminium production.
The terms voltage drop, conductivity, resistance and current efficiency are used interchangeably in the following as it is found natural and are used in general by skilled persons. It is assumed that skilled persons know the relationship between the terms, for example by the Ohm's law and Faraday's law for electrolysis, and know how the terms are interrelated with the problem of the present invention.
In cells for electrolysis of alumina for production of aluminium it is today utilized in general two main types of anodes, namely the so called prebaked anodes and anodes of the Söderberg type. It also exists non-carbon anodes and non-carbon cathodes which are relevant for utilization with the present invention, but these have so far no or little utilization and will therefore not be considered specifically. The anodes are usually formed of carbon with an inner current bus bar, namely anode hangers and anode bolt (anode stud bolt), whereto current is applied. The current is passed from said current conducting devices through the carbon of the anode and into the electrolyte where electrolysis takes place, and further into the cathode, optionally first through a layer of melted aluminium on the cathode, and to the current conduction devices of the cathode, and from there for example in series to the next electrolysis cell.
Voltage drop appears all over the electrolysis cell, of which the most significant voltage drop takes place over the electrolyte. However, voltage drop also appears to the current conducting devices, which means the current bus bars of the anodes, namely the anode hangers and the anode bolts, and current bus bars of the cathodes, Taking into account that the amount of current through a typical electrolysis oven of today for production of aluminium is between 100000 and 300000 ampère, even a small reduction of the voltage drop will be very significant.
In the devices for conducing current at present materials as iron or steel are used, optionally with outer parts of copper or aluminium, and the design is so that the voltage drop is to be minimized. For a simple description, it is by the term steel in the following considered both iron- and steel alloys.
The current bus bars of the cathode are at present manufactured form massive steel in the part which is to be incorporated into the cathode, optionally with ends extending from the electrolysis cell, which ends are of another material with better conductivity, for example copper. The part of the anode hangers or anode bolts which is to be incorporated into the carbon is at present manufactured from steel, while the upper, upwardly extending part via a bimetal transition is manufactured from aluminium. The devices of today contain several welds, usually manual welds performed in difficult welding positions, with resulting poor quality with low conductivity and strength for example, the bimetal transition results in three welds, namely a manual above and a manual below, in addition to the bimetal welding which is roll-welded at high temperature and high pressure.
In practical utilization at present in anodes and cathodes of carbon, in the a within and close to the electrode body (electrode mass), it is utilized current bus bars of massive steel. Efforts have been taken to replace this material with better conducting materials closer towards the electrolysis bath, which in practice has been very difficult. In patent publication NO 162083 description is found on an anode hanger for holding a carbon containing anode in cells for production of aluminium. According to said publication the anode hanger for holding a carbon containing anode in cells for production of aluminium by electrolysis of the melt according to the Hall-Heroult-process, consists of an upper part of a metal such as aluminium, copper or steel, which is joined by an anode beam or something corresponding, and a lower current conducting steel part which is fastened to the upper part and which comprises a yoke with downward extending nipples whereto the carbon containing anode is secured, and said anode hanger is in particular distinguished in that the upper part is fastened to the lower current conducting steel part by means of a cast-joining of aluminium or copper. In practice the yoke according to NO 162083 is produced by filling a void in the yoke by melted aluminium which then solidifies and makes the inner part of the yoke, which thereby is supposed to be a better conductor, However, the anode hanger according to the above publication has by experience appeared not to be industrially applicable, by several reasons. More specific it has been observed that the joining between the cast aluminium and the steel has not sufficient mechanical strength under the harsh conditions to withstand the thermal expansion. The components are disintegrated, in particular the joining steel/aluminium, the carbon around the nipples is breaking up and the carbon can fall down into the electrolyte (“cowboy”). An uneven current conduction appears both by the known devices and the anode hanger according to NO 162083, indicated by non-uniform carbon deterioration. Despite significant efforts to provide improvements with respect to reduced voltage drop, so far it has not been possible to provide devices which are industrially applicable. Poor thermal conductivity is also a problem with the prior art devices. Accordingly, a significant demand for improvements exists.